As is known in the art, dielectric layers are frequently used in contact with metal. One such application is in capacitors such as those used in integrated circuits, such as monolithic microwave integrated circuits (MMICs). In such MMICs active devices may be connected to capacitors. One such capacitor is a metal-insulator-semiconductor (MIS) capacitor. One such capacitor structure used in a GaN MMIC is shown in FIG. 1. Here a layer of gallium nitride is disposed on an insulating substrate such as silicon, gallium arsenide, or silicon carbide. The capacitor includes a lower electrode made up of a lower layer of titanium, a layer of platinum on the lower layer of titanium, and a relatively thick, for example, 1500 nm thick, layer of gold on the layer of platinum. A dielectric film, such as for example, silicon nitride is used as the dielectric for the capacitor. The upper electrode for the capacitor includes a lower layer of titanium on the dielectric layer, a layer of platinum on the layer of titanium, and a relativity thick, for example, 1000 nm thick, layer of gold on the layer of platinum. It is noted that the bottom metal can be Ti/Au or just Au.
As is also known in the art, there are a number of process steps that have temperature cycles. For example, SiN deposition is about 300 degrees C. and each photolithographic step can be 150 degrees C. or so; there are also typically stabilization bakes at the end of the process; and finally, the mounting process (solder) is typically 280-320 C.
We have noted that the thermal expansion coefficient of the gold is much larger that the thermal expansion coefficient of the GaN, and, as a result, the ductile gold is irreversibly deformed during thermal cycling. The exact shape of the deformation depends on the shape of the capacitor and its location on the die, but the edge of the gold film will often slope inward after thermal cycling stressing the dielectric film that covers that edge. Our examination of SEM photos of cracks in the capacitor dielectrics shows that this is usually what happens with the observed cracks originating at the edge of the gold film.
Prior attempts had involved changing the shape and the layout of the capacitors (rounding corners, etc.). These changes helped, but did not eliminate the cracking.
In accordance with the present invention, a structure is provided comprising: a dielectric layer; and a metal structure deposed in contact with the dielectric layer. The metal structure comprises: a first metal layer; a refractory metal layer disposed in contact with the first metal layer; and a third metal layer disposed in contact with the second metal layer.
In one embodiment, the first metal layer is a metal in a group consisting of gold, silver, copper and aluminum.
In one embodiment, the third metal layer is a metal in a group consisting of gold, silver, copper and aluminum.
In one embodiment, the refractory metal is a group consisting of titanium, tungsten, molybdenum, and chromium.
In one embodiment, a semiconductor structure is provided having: a substrate; a dielectric layer disposed over the substrate; and a metal structure in contact with the dielectric layer. The metal structure comprises: a first metal layer; a second metal layer disposed in contact with the first metal layer, the second metal layer being stiffer than the first metal layer; a third metal layer disposed under, and in contact with the second metal layer, the second metal layer being stiffer than the third metal layer.
In one embodiment, the first and third metal layers are gold and the second metal is titanium. With such a structure, thin layers of titanium distributed through the gold are included to stiffen the gold layer and to reduce the permanent deformation produced by thermal cycling. Reducing the deformation of the bottom electrode eliminated the cracking of the dielectric. Thus, we have recognized that cracking during thermal cycling of dielectric films deposited on the metal films normally used in microelectronic structures can be eliminated if a stiffened metal film is substituted for the ductile metal films (e.g., pure copper, aluminum, silver or gold) that are typically used. One method of stiffening a ductile metal film (such as pure gold) while retaining its electrical conductivity is to alternate layers of the ductile metal with layers of a stiff metal (such as titanium, chromium, or molybdenum); that is with a metal stiffer than the ductile metal.
In one embodiment, an additional metal is included wherein the dielectric layer is disposed between the metal structure and the additional metal.
In one embodiment, the additional metal comprises a second metal structure and wherein the second metal structure comprises; a fourth metal layer; a fifth metal layer disposed on, and in contact with the fourth metal layer, the fifth metal layer being stiffer than the fourth metal layer; a sixth metal layer disposed on, and in contact with the fifth metal layer, the fifth metal layer being stiffer than the sixth metal layer.
In one embodiment, the second metal layer is a refractory metal.
In one embodiment, the first metal layer is a metal in a group consisting of gold, silver, copper and aluminum.
In one embodiment, the third metal layer is a metal in a group consisting of gold, silver, copper and aluminum.
In one embodiment, the refractory metal is a group consisting of titanium, tungsten, molybdenum, and chromium.
In one embodiment, a capacitor is provided. The capacitor includes: a dielectric layer; a metal structure deposed in contact with the dielectric layer. The metal structure comprises: a first metal layer; a second metal layer disposed on, and in contact with the first metal layer, the second metal layer being stiffer than the first metal layer; a third metal layer disposed on, and in contact with the second metal layer, the second metal layer being stiffer than the third metal layer; and an additional metal, wherein the dielectric layer is disposed between the metal structure and the additional metal.
In one embodiment, the capacitor includes an additional metal structure and wherein the second metal structure comprises; a fourth metal layer; a fifth metal layer disposed on, and in contact with the fourth metal layer, the fifth metal layer being stiffer than the fourth metal layer; a sixth metal layer disposed on, and in contact with the fifth metal layer, the fifth metal layer being stiffer than the sixth metal layer.
In one embodiment, the second metal layer of the capacitor is a refractory metal.
In one embodiment, the first metal layer of the capacitor is a metal in a group consisting of gold, silver, copper and aluminum.
In one embodiment, the third metal layer of the capacitor is a metal in a group consisting of gold, silver, copper and aluminum.
In one embodiment, the refractory metal of the capacitor is a group consisting of titanium, tungsten, molybdenum, and chromium.
In accordance with another feature of the invention, a method is provided for reducing cracks in a dielectric layer. The method comprises: depositing a metal structure in contact with the dielectric layer, such metal structure comprising: a first metal layer; a refractory metal layer disposed on, and in contact with the first metal layer; a third metal layer disposed on, and in contact with the second metal layer.
In one embodiment, the first metal layer is a metal in a group consisting of gold, silver, copper and aluminum.
In one embodiment, the third metal layer is a metal in a group consisting of gold, silver, copper and aluminum.
In one embodiment, the refractory metal is a group consisting of titanium, tungsten, molybdenum, and chromium.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.